As the contact lens, there have been mainly used hard contact lenses composed mainly of poly(methyl methacrylate) and soft contact lenses obtained by polymerizing a hydrophilic monomer such as 2-hydroxyethyl methacrylate or N-vinylpyrrolidone.
However, it has been found that these materials involve problems as described below. More specifically, a hard contact lens composed mainly of poly(methyl methacrylate) is excellent in the mechanical strength, the transparency and the astigmatism-correcting capacity, but it is defective in that it has no substantial oxygen permeability and the wearing time is limited because of a feeling of a foreign substance.
A soft contact lens formed by using a hydrogel (see, for example, U.S. Pat. No. 4,136,250) is characterized in that it has an oxygen permeability and since it is soft, a feeling of foreign substance is not given during wearing, but it is more readily broken than a hard contact lens and since components of tear adhere to the soft contact lens, it should be periodically washed. Furthermore, since the soft contact lens contains water, it should be sterilized.
Recently development of an oxygen-permeable hard contact lens having merits of both the conventional hard and soft contact lenses has been tried. For example, such a hard contact lens is disclosed in U.S. Pat. No. 4,327,203. A silicone-type polymer is well-known as an oxygen-permeable material. However, this polymer involves the following problems. In the first place, since the polymer is poor in mechanical properties, a contact lens prepared from this polymer is readily broken. In the second place, since the polymer is water-repellent, if a contact lens of this polymer is worn, no tear layer is formed between the cornea and the contact lens, a feeling of a foreign substance is given to a wearer and adhesion of the contact lens to the cornea is caused.
As means for solving these problems involved in the silicone-type polymer, there has been adopted, for example, copolymerization with a monomer giving a polymer having a hign glass transition temperature or graft copolymerization of a hydrophilic monomer to the surface. However, it has been difficult to simultaneously attain a high oxygen permeability and good performances required for a hard contact lens, such as high strength, mechanical processability, surface water wettability and transparency. Namely, if the oxygen permeability is improved, the strength or surface water wettability is reduced, and if the strength is improved, the oxygen permeability is reduced.
As the prior art closest to the present invention, there can be mentioned Japanese Patent Application No. 58-190617 (U.S. patent application Ser. No. 433,743 filed on Oct. 12, 1982). However, this prior art is still insufficient in some points. For example, if a diisocyanate compound is used, the number of urethane bonds is small and the elasticity is poor. Furthermore, if a triisocyanate compound is used, gelation is readily caused at the step of synthesizing a polysiloxane macromer and copolymerization with a water-soluble monomer or a water-insoluble monomer is impossible, and a stable polymer cannot be obtained.